When steels of a high carbon content such as tool steel are molten in an electric furnace and cast into an ingot case, carbides are crystallized out from the melt and they are likely to become coarse with a non-uniform distribution. Therefore, it is generally impossible to obtain required quenching and mechanical properties. It has been conducted to subject an ingot to the homogeneous heat treatment and hot processing to finely divide carbides and distribute them uniformly. Even if such treatment is conducted, it is difficult for the treated product to exert properties inherent of the material sufficiently. Especially when the ingot is large, formation of coarse carbides is enhanced, resulting in extreme reduction of the abrasion resistance or toughness.
Accordingly, a sintered body prepared by molding and sintering a powder formed by spraying a molten steel has been investigated. When a molten steel is sprayed into a spray medium, since fine particules of the sprayed metal are rapidly cooled, carbides formed at the solidification of the metal are very fine and dispersed uniformly in the solidified metal. If it is possible to prepare a sintered product having a density approximating the theoretical density by molding the so formed powder into a desired form and hot pressing it at such a high temperature as ranging from 900.degree. to 1200.degree.C., a sintered product having very excellent properties will probably be obtained.
Problems involved in the powder metallurgy are the oxygen content in the metal powders and the shape of the metal powders. Because of a large surface area of metal powders, even in the case of a thin oxide film, the total amount of oxygen contained in the powders is extremely high. Especially when a component having a high affinity with oxygen such as chromium or vanadium is contained in the metal, the oxygen content tends to increase, and the toughness of the material is readily degraded, with the result that the effect owing to dispersing carbides finely and uniformly is lost. For example, the impact value of a high speed steel manufactured by power metallurgy (corresponding to JIS SKH 57 steel or JIS BS BT 42 steel) varies greatly depending on the oxygen content, and although at an oxygen content not exceeding 100 ppm the impact value is higher than 2 kg-m/cm.sup.2, at an oxygen content exceeding 200 ppm the impact value is reduced below 1.7 Kg-m/cm.sup.2. A high speed steel of the same composition prepared by melting has a low oxygen content of about 50 ppm owing to the coarse size and non-uniform distribution of carbides, but its impact value is as low as about 1 Kg-m/cm.sup.2.
An irregular shape is suitable for molding by powder metallurgy, and a metal powder having a globular or drop-like shape is poor in moldability and it cannot be used as it is. However, when a spherical powder is mechanically pulverized so as to change its form into irregular one, incorporation of impurities or pollution of air by dust is caused to occur. Therefore, it is desired that such mechanical pulverization method is not adopted.
In the atomizing method for preparing a metal powder, it is known that a metal powder of such a low oxygen content as about 100 ppm can be prepared by employing argon gas as a spray medium and conducting the atomizing process in an inert gas atmosphere. However, this technique involves a difficulty in attainment of an air-tight structure and a problem of a high manufacturing cost caused by consumption of argon gas. Further, this technique is fatally defective in that the resulting powder has a spherical shape and hence, is poor in moldability. In case a gas is used as a spray medium, since the volume of the gas is greatly expanded on departure from a spray nozzle, a high effect of pulverizing molten metal cannot be obtained and the resulting powder has a size distribution including large quantities of coarse particles. Further, since the inert gas has generally a low specific heat, it takes a long time for liquid drops of the molten metal to be solidified and they tend to have a spherical shape. Simultaneously, crystals of metal grow during solidification and the composition of precipitates differs between the inner layer and outer layer of the metal particle.
As noted above, atomized powders formed with use of inert gases have an advantage of a low oxygen content, but they are defective in other various points and they are not suitable for practical use.
When water is used as a spray medium in the atomizing method, because of its high activity of finely dividing molten metal and its high specific heat, water gives fine metal particles irregular in the shape while exhibiting a rapid cooling effect. In customary water atomizing methods, the average dendrite arm spacing is almost constant and about 1 .mu.m in metal particles having a particle size of 50 to 300 .mu.m. This feature is very advantageous when it is intended to prepare a sintered body having a uniform texture, but because of occurrence of the reaction between molten metal and water, the oxygen content is increased in the resulting metal powder. It has heretofore been impossible to prepare a metal powder of an oxygen content lower than 1500 ppm according to the water atomizing method. Accordingly, methods of reducing the oxygen content of 2000 ppm or higher in metal powders prepared according to the water atomizing below 500 ppm have been proposed and conducted. For instance, there have been conducted methods in which a high carbon content alloy such as tool steel is prepared by heating metal powder in advance in a hydrogen atmosphere or incorporating into metal powder an excessive amount of carbon, pre-molding the powder under compression and heating it in vacuum at 900.degree. to 1250.degree. C. for a long time thereby to reduce oxygen bonded to the metal powder surface with excessive carbon.
However, when such heat treatment is conducted for a long time, carbides grow and agglomerate, and therefore, the inherent advantages attained by adoption of the sintering method are greatly reduced. Further, in some cases, the carbon content differs between the surface layer and inner layer of the resulting sintered body and it is impossible to obtain a product having a uniform composition. Moreover, in the above reducing treatment, it is necessary to maintain the metal powder in a prescribed atmosphere until the metal powder is cooled completely to room temperature, with the result that it is impossible to increase the production efficiency.
Accordingly, development of a process that can provide metal powders of a low oxygen content while utilizing fully advantages of the water atomizing method has been greatly demanded in the art.